Atmospheric aerosols from natural, anthropogenic, and industrial sources have long been recognized as a potential threat to human health. This threat is now compounded by the need to detect and avert acts of terrorism where an infectious or toxic material is deployed in the form of an aerosol. Particles that present the greatest hazard in terms of inhalation and nasal entrapment or lung deposition are respirable particles in the range of 0.02-25 um diameter.
One major challenge that must be addressed by all aerosol samplers is that many aerosols occur at extremely low concentrations, or may be only a small fraction of the urban background aerosol distribution. The aerosol must thus generally be concentrated before sampling. Convergent nozzles and aerodynamic lenses are effective in focusing an aerosol into a beam of particles, a particle-rich core surrounded by a sheath of particle-depleted gas. A discussion of focusing aerosols is found in U.S. Pat. No. 5,565,677 to Wexler and in Peng et al. (1995) Aerosol Sci Technol 22:293-313. But used in isolation such focusing devices are not effective in fractionating the particle-rich core from the particle-depleted sheath flow.
For concentration of particles, a device which may be used in conjunction with nozzles or aerodynamic lenses is a “virtual impactor,” which separates particles from carrier gas on the basis of momentum and aerodynamic size [See Loo et al. Dichotomous virtual impactors for large scale monitoring of airborne particulate matter, In (B Y H Liu, ed) Fine Particles: Aerosol generation, measurement, sampling and analysis (1976) pp 312-349]. A virtual impactor does not trap particles by physical impaction, as in plate impactors or impingers, but instead fractionates a particle beam according to a “cut size” characteristic of the virtual impactor, fractionating the gas stream into flows that are termed by convention, the “bulk flow”, which is the particle-depleted sheath flow, and the “minor flow”, which is the particle-rich core flow of the focused gas stream. Both bulk and minor flows generally flow in the direction of a suction pressure applied to the concentrator. Particles in the minor flow are concentrated in the virtual impactor and remain suspended in a reduced volume of flowing gas. The bulk flow is routed to an exhaust manifold. This function has the advantage that virtual impactors can be operated continuously.
In a virtual impactor, the impaction plate or impinger is replaced by a column of lower velocity gas occupying what is termed a “virtual impact void”. The particle-rich core of the particle beam collides with this column of lower velocity gas. The bulk or sheath flow is diverted around it. Historically, this is accomplished by inserting a tubular, wedge-shaped, or conical “collection nose” (also termed a “collection probe”) into the flow of the gas stream. The collection nose is commonly acutely tapered at the tip to split off and deflect the sheath flow, and is formed with a tubular channel down its center axis. The mouth of that channel is the virtual impact void. Particles continue through the virtual impact void and are carried in the lower velocity gas stream (the “minor flow”) down the channel in the nose (conventionally termed the “minor flow channel” but termed here the “collector”). The coarse particles, with greater inertia, pass into the collector, and in contrast, the sheath flow and some finer particles follow the streamlines of the bulk air flow and are diverted by the outer surface of the nose. In this way, the gas flow is fractionated; the bulk of the gas is diverted away from the nose, and a lesser, particle-rich fraction is concentrated in the collector flow. Because the nose of these virtual impactors generally comes to a sharp tip, it is typically manufactured by machining, or is sacrificially truncated.
Representative virtual impactors are found in U.S. Pat. Nos. 3,901,798, 4,301,002, 4,670,135, 4,767,524, 5,425,802, 5,533,406, 5,788,741, 6,062,392, 6,386,015, and 6,402,817. Because typical virtual impactors do not actually collect particles themselves, but merely redirect them into two different fluid streams according to their mass, they are essentially free of the problems of particle bounce and re-entrainment associated with actual impactor devices. Related designs are described in Chen, B T and H C Yeh (1985) A Novel Virtual Impactor: Calibration and Use, J Aerosol Sci 16: 343-354; in Novick V S and J L Alvarez (1987) Design of a multi-stage virtual impactor, Aerosol Sci Tech 6:63-70; in Loo B W and C P Cork (1988) Development of high efficiency virtual impactors, Aeros Sci Techn 9:167-176; in Marple V A et al (1980) Virtual Impactors: a theoretical study, Environ Sci Tech 14:976; and in Goo, J (2002) Numerical simulation of aerosol concentration at atmospheric pressure by a cascade of aerodynamic slit lenses, J Aerosol Sci 33:1493-1507. As is the case with solid body impactors, parameters used to characterize the performance of virtual impactors include collection efficiency, separation efficiency, wall loss, volume per unit time, flow split, or concentration ratio. A preferred virtual impactor should have a steep cutoff curve and little wall loss, and is preferentially operated at a larger mass transfer rate and flow split.
Typically a virtual impactor is described as having a specific “cut size”. This term refers to the particle size above which a given particle has sufficient momentum to cross deflected streamlines and flow into the collector. In contrast, particles smaller than the cut size and carrier gas molecules follow their respective streamlines. By convention, the cut size is determined by measuring the particle size at which 50 percent of the particles of that particular size flow into a collection tube and 50 percent of the particles of that size follow the deflected streamlines. The “efficiency” of a virtual impactor for a particular particle size is the percentage or ratio of correctly fractionated particles over the total number of particles in the sampled gas stream. Efficiency values for different particle sizes are not only indicative of the cut size value, but can also indicate the overall effectiveness of the virtual impactor for other particle sizes. The “particle loss” for a virtual impactor represents the percentage of particles that enter but do not exit the device, and instead adhere to some internal structure, such as the acceleration nozzle, the collection probe inlet, or walls of the collector.
Peterson, in U.S. Pat. No. 3,901,798, claims improvements in reduction in wall loss by stacking two plates, the first plate and the second plate having concentrically located orifices, with a tubular collection nose inserted into the gas flow in the plane of the second orifice so that an annular passage for diversion of the bulk flow (B) is formed around the tubular nose. Minor flow (M) exits at the long center axis. The virtual impactor in this early design is not preceded by a focusing nozzle. This early design of a virtual impactor, represented here in FIG. 1, is not readily scaleable for handling large air flows in a compact device and is expected to have a relatively high cutoff size.
Loo, in U.S. Pat. No. 4,301,002 describes a focusing nozzle and collection probe of a virtual impactor, shown here in FIG. 2. A single air pump provides suction for the device, which measures 45 to 115 cm in height. The tubulation upstream of the virtual impactor (1), in combination with the pendant conical flow acceleration nozzle (termed a “jet orifice,” 2) immediately above the conical nose (3) of the virtual impactor, is taught to focus a particle rich core flow (M) and divert the bulk flow (B) around the conical nose. The orifice diameter of the jet orifice is slightly larger than the orifice diameter of the virtual impact void inlet, reducing wall losses. Distance (S) separates the pendant cone (2) of the acceleration nozzle from the conical nose (3) of the virtual impactor. Loo teaches that the impact void nose should be radiused (R) and tapered to smooth separation of the flow streamlines in the separation region. Filters are used to trap particles in the outlet stream. This geometry, however, is associated with stagnation points and eddy instability at the point of flow separation that leads to particle loss by wall collisions and diversion with the bulk flow.
Marple, in U.S. Pat. No. 4,670,135 (FIG. 3) addresses the problem of scaling virtual impactors for higher gas flow sampling rates. In this device, multiple virtual impactors, operated in parallel, and consisting of pendant acceleration nozzles (4) and tubular receiving noses or probes (5), are assembled into larger manifolds. Again, filters are used to collect particles in the bulk (B) and minor flow (M) streams.
Burton (FIG. 4), in U.S. Pat. No. 5,425,802 and U.S. Pat. No. 5,788,741, describes a now familiar construction of acceleration nozzle and conical collection nose (6) with axial virtual impact void. Two vacuum sources are used, allowing the investigator to vary the ratio of the suction pressure applied to the bulk (B) and minor flow (M) channels. However, difficulties are reported due to impaction losses on the cone of the collection probe and to eddying around the virtual impact void.
Kenning in U.S. Pat. No. 6,290,065 describes in (FIG. 5) a virtual impactor with tapered inlet nozzle (7,8), central minor flow passage (collector 9), outlet for collecting a minor flow (M), and lateral flow channels (13) for diverting a bulk flow. Several modules 11 are shown in a one-dimensional row (10). The lateral channels terminate in “major flow ports” on the plane of the paper (12).
A related device is shown in FIG. 6. Minor flow (M) and bulk flow (B) are separated at a virtual impactor (16) formed of “fin-shaped projections” (24), inner walls (26), and minor flow channel (30), the combination forming a virtual impactor body (33). According to Kenning, the invention excludes or is not inclusive of two-dimensional stacks of these sort of devices. Kenning reports, “By improving the particle separation efficiency of each of virtual impactors (16), the present invention allows for employing only one layer or row of virtual impactors (16) for completing particle separation, which eliminates the chances of particles getting lost onto surfaces of additional layers or rows of virtual impactors” (Col 6, lines 1-7 of U.S. Pat. No. 6,290,065). Construction of these devices, particularly the acute angles of the nose, relies on difficult and expensive micromachining techniques.
Similarly, Birmingham, in U.S. Pat. No. 6,062,392, describes separation plates containing linear arrays of acceleration nozzles and “fin-shaped” virtual impactor noses. Importantly, the sharply convex shape of the collection nose is taught to reduce wall losses in this design. Birmingham teaches, “The virtual impactor is generally haystack-shaped and includes a convex leading surface. The convex surface faces the outlet end of the nozzle. The convex surface includes a virtual impact void therethrough. The virtual impact void defines a terminal end of a minor flow channel that extends through the separation plate to the second surface.” (Col 3, lines 58-63). And further that a “dead fluid” zone or a zone of stagnant air is created adjacent to the convex surfaces surrounding the virtual impact void, the convex surfaces permitting improved collection of the minor flow (Col 6, lines 18-25). This description is consistent with wall separation, which is accompanied by instability in the flow regime around the nose, and is not expected to result in higher efficiencies at higher flow rates and flow splits.
Bulk flow is diverted to ducts interposed between the minor flow channels and from there out through orifices in the coverplates of the device. These separation plates, however, cannot be stacked because of mechanical interferences, and because pipeflow resistances rapidly lead to a decrease in pressure drop in the bulk flow exhaust from one layer to the next. According to Birmingham, “By improving the particle separation efficiency of each of virtual impactors 16, the present invention allows for employing only one layer or row of virtual impactors 16 for completing particle separation, which eliminates the chances of particles getting lost onto surfaces of additional layers or rows of virtual impactors” (Col 6, lines 38-42). Construction of these devices also relies on difficult and expensive micromachining techniques.
In contrast to the single-layered devices of Birmingham and Kenning, Ariessohn's “Aerodynamic Lens Particle Separator” (US 20080022853), describes an expandable two-dimensional array of micro-aerodynamic lenses for focusing large volumes of moving air into concentrated particle beams. FIG. 7 is a conceptual model used for illustration of a virtual impactor and skimmer used in conjunction with an upstream aerodynamic lens. Bulk flow (B) in the ADL-skimmer element is directed into lateral flow channels perpendicular to the long axis of flow of the gas stream in the accelerator nozzle, shown here in section with aerodynamic lens ringlets (34) adorning the throat of the skimmer. See also FIG. 8 of the published application. US 20080022853 is co-assigned and is herein incorporated in full by reference.
Arrays of the devices of FIG. 7, like those of Birmingham and Kenning, are not readily assembled by joining individual modules. In multiple layers, the lateral flow channels would necessarily be connected in series, increasing resistance with depth of the array, and resulting in degraded performance. Also, there is no provision for combining collector flows downstream from multiple devices in an array.
Related art is described by Goo (Goo J. 2002. Numerical simulation of aerosol concentration at atmospheric pressure by a cascade of aerodynamic slit lenses. J Aerosol Sci 33:1493-1507). FIG. 8 of the reference describes computer modeled streamline flow in a skimmer having orthogonally directed lateral flow channels. Eddies in the throat of the skimmer arms impinge on the long axis of flow of the particle beam. Also seen is wall separation in the streamlines branching laterally. When similar geometries are tested experimentally, particle capture efficiency decreases are noted, due to loss of particles to the bulk flow and due to collision of particles with the walls around the mouth of the virtual impactor.
Thus, there is a need in the art, for a virtual impactor module that overcomes the above disadvantages, is readily manufactured without recourse to micromachining, and can be scaled or assembled in arrays to accommodate larger flow throughputs at high ratios of bulk flow to minor flow.